Hydraulic fracturing has been widely used as a means for improving the rates at which fluids can be injected into or withdrawn from subterranean formations surrounding oil wells and similar boreholes. The methods employed normally involve the following steps: injection of a viscous fracturing fluid having a low fluid loss value into the well at a rate sufficient to generate a fracture in the exposed formation, the introduction of fluid containing suspended propping agent particles into the resultant fracture, and the subsequent shutting in of the well until the formation is closed on the injected particles. This results in the formation of vertical, high-conductivity channels through which fluids can thereafter be injected or produced. The conductivity in the propped fracture is the function of the fracture dimensions and the permeability of the bed of propping agent particles within the fracture.
In order to generate a fracture of sufficient length, height, and width and to carry the propping agent particles into the fracture, it is necessary for the fluid to have relatively high viscosity. This requires the use of gelling agents in the fracturing fluid.
Fracturing fluids are generally water or oil-based liquids gelled with polymers. However, the water-based liquids cannot be used in the fracturing of many water sensitive formations. Fracturing in these formations requires the use of oil-based liquids.
The most common gelling agent for oil-based fracturing fluids has been aluminum salts of phosphate esters. The following references discloses phosphate ester gelling agents:
(a) U.S. Pat. No. 4,104,173 discloses gelling agents prepared by PA1 (b) U.S. Pat. No. 4,316,810 discloses gelling agents prepared by PA1 (c) U.S. Pat. No. 4,787,994 discloses the use of an ail with a gelling agent prepared by reacting an orthophosphate ester with an aluminum activator. The acid is mixed with the orthophosphate ester prior to the addition of the activator. PA1 (d) U.S. Pat. No. 4,877,894 and 5,057,233 discloses gelling agents prepared by PA1 (e) U.S. Pat. No. 5,190,675 discloses a metal phosphate diester prepared by reacting a triester phosphate with P.sub.2 O.sub.5 to form a polyphosphate which is reacted with an alcohol to produce a phosphate diester. The diester is then contacted with an aluminum source (in the hydrocarbon liquid) to form the aluminum phosphate diester. PA1 (f) U.S. Pat. No. 5,202,035 discloses a gelling agent similar to that disclosed in 5,057,233, except that an aluminum sulfate was used as a mixture with the aliphatic alcohol. PA1 (g) U.S. Pat. No. 5,514,645 discloses a gelling agent comprising two components to be added to the liquid hydrocarbon: PA1 (h) U.S. Pat. Nos. 5,417,287; 5,571,315; 5,614,010; and 5,647,900 disclose organic phosphate gelling agents and (i) ferric salts, or (ii) ferric ammonium citrate, or (iii) ferric ions, an amine, and polycarboxylic acid, or (iv) a ferric salt reacted with a low molecular weight amine. PA1 (i) U.S. Pat. No. 4,200,540 discloses gelling agents made from reacting low molecular weight alcohols with phosphorous pentoxide, which in turn is reacted with a basic aluminum compound. PA1 (a) a high molecular weight gelling agent additive; PA1 (b) a low molecular weight phosphate ester acid; and PA1 (c) a Group II or Group III metal salt.
(i) reacting phosphorous pentoxide with monohydric alcohols and PA2 (ii) reacting this product with a basic aluminum compound. PA2 (i) reacting phosphorous pentoxide with ethereal alcohols prepared by reacting ethylene oxide or propylene oxide with selected aliphatic alcohols to form a phosphate ester and PA2 (ii) reacting phosphate ester with an aluminum compound to form the aluminum salt of the phosphate ester. PA2 (i) reacting triethyl phosphate with phosphorous pentoxide to form a polyphosphate intermediate, PA2 (ii) reacting the polyphosphate intermediate with a mixed aliphatic alcohol. This product is then reacted with an aluminum activator to form the aluminum alkylphosphate ester gelling agent. PA2 (i) an alkyl orthophosphate acid ester partially neutralized with an alkali metal hydroxide, and PA2 (ii) a source of aluminum or ferric ions.
One of the problems associated with oil based gelling agents is that they generate viscosity slowly, which means that the viscosified oil must be premixed by a batch process. In some fracturing operations, it would be highly desirable and economical to carryout the fracturing operations by a continuous process wherein the oil is viscosified "on the fly." The continuous operation requires the use of a fast acting gelling agent.